HEIC0303: EMBARGOED UNTIL: 20:00 (CET) 12 March, 2003
News release:
European astronomers observe first evaporating planet
12-Mar-2003 Using the Hubble Space Telescope, for the first time,
astronomers have observed the atmosphere of an extrasolar planet
evaporating off into space. Much of this planet may eventually
disappear, leaving only a dense core. The planet is a type of
extrasolar planet known as a ‘hot Jupiter’. These giant, gaseous
planets orbit their stars very closely, drawn to them like moths to a
flame.
The scorched planet called HD 209458b orbits ‘only’ 7 million
kilometres from its yellow Sun-like star. By comparison, Jupiter, the
closest gas giant in our Solar System, orbits 780 million kilometres
from our Sun. The NASA/ESA Hubble Space telescope observations reveal a
hot and puffed-up evaporating hydrogen atmosphere surrounding the
planet. This huge envelope of hydrogen resembles a comet with a tail
trailing behind the planet. The planet circles the parent star in a
tight 3.5-day orbit. Earth also has an extended atmosphere of escaping
hydrogen gas, but the loss rate is much lower.
A mainly European team led by Alfred Vidal-Madjar (Institut
d’Astrophysique de Paris, CNRS, France) is reporting this discovery in
the March 13 NATURE Magazine. "We were astonished to see that the
hydrogen atmosphere of this planet extends over 200 000 kilometres,"
says Vidal-Madjar.
Studying extrasolar planets, especially if they are very close to their
parent stars, is not very easy because the starlight is usually too
blinding. The planet was also too close to the star for Hubble to
photograph directly in this case. However, astronomers could observe
the planet indirectly since it blocks light from a small part of the
star during transits across the disk of the star, thereby dimming it
slightly. Light passing through the atmosphere around the planet is
scattered and acquires a signature from the atmosphere. In a similar
way, the Sun’s light is reddened as it passes obliquely through the
Earth’s atmosphere at sunset. Astronomers used Hubble’s Space Telescope
Imaging Spectrograph (STIS) to measure how much of the planet's
atmosphere filters light from the star. They saw a startling drop in
the star's hydrogen emission. A huge, puffed-up atmosphere can best
explain this result.
What is causing the atmosphere to escape? The planet’s outer atmosphere
is extended and heated so much by the nearby star that it starts to
escape the planet's gravity. Hydrogen boils off in the planet’s upper
atmosphere under the searing heat from the star. "The atmosphere is
heated, the hydrogen escapes the planet's gravitational pull and is
pushed away by the starlight, fanning out in a large tail behind the
planet - like that of a comet," says Alain Lecavelier des Etangs
working at the Institut d’Astrophysique de Paris, CNRS, France.
Astronomers estimate the amount of hydrogen gas escaping HD 209458b to
be at least 10 000 tonnes per second, but possibly much more. The
planet may therefore already have lost quite a lot of its mass.
HD 209458b belongs to a type of extrasolar planet known as ‘hot
Jupiters’. These planets orbit precariously close to their stars. They
are giant, gaseous planets that must have formed in the cold outer
reaches of the star system and then spiralled into their close orbits.
This new discovery might help explain why ‘hot Jupiters’ so often orbit
a few million kilometres from their parent stars. They are not usually
found much closer than 7 million kilometres, as is the case for HD
209458b. Currently, the current closest distance is 5.7 million
kilometres. Hot Jupiters have orbits that are as brief as 3 days, but
not shorter. Perhaps the evaporation of the atmosphere plays a role in
setting an inner boundary for orbits of hot Jupiters.
# # #
Notes for editors
HD 209458b has a diameter 1.3 times that of Jupiter, and two-thirds the
mass. Its orbit is one-eighth the size of Mercury's orbit around the
Sun. The parent star is similar to our Sun and lies 150 light-years
from Earth. It is visible with binoculars as a seventh magnitude star
in the constellation of Pegasus. In 1999, this star suddenly entered
the astronomical Hall of Fame when the extrasolar planet HD 209458b
passed in front of it and partly eclipsed it. This was the first
confirmed transiting extrasolar planet ever discovered. In 2001, Hubble
detected the element sodium in the lower part of HD 209458b’s
atmosphere, the first signature of an atmosphere on any extrasolar
planet.
The team is composed of A. Vidal-Madjar, lead author of the discovery,
(Institut d’Astrophysique de Paris, CNRS, France) A. Lecavelier des
Etangs, J.-M. Désert (Institut d'Astrophysique de Paris, CNRS, France),
G. Ballester (University of Arizona, United States), R. Ferlet and G.
Hébrard (Institut d'Astrophysique de Paris, France), and M. Mayor
(Geneve Observatory, Switzerland). They observed three transits of the
planet in front of the star with Hubble. The observations of the atomic
hydrogen envelope were made in ultraviolet (Lyman-alpha) light, using
Hubble’s spectrograph STIS. Hubble’s position above the atmosphere makes
it the only telescope currently that can perform these types of
ultraviolet studies.
For broadcasters, computer animations of the discovery plus general
Hubble Space Telescope background footage is available from the ESA
Television Service, see http://television.esa.int
The search and the study of extrasolar planets is the aim of several of
ESA’s scientific missions. Eddington, for instance, due for launch in
2007, will discover large numbers of transiting planets of all types,
including many transiting ‘hot Jupiters’ similar to HD 209458b. These
will be ideal targets for the same type of detailed follow-up studies
with large space- and ground-based telescopes.
This material is being co-released with NASA/STScI/OPO.
Image credit: European Space Agency, Alfred Vidal-Madjar (Institut
d’Astrophysique de Paris, CNRS, France) and NASA.
For more information, please contact:
ESA Communication Department
Media Relations Office, Paris, France
Tel: +33(0)-15369-7155
Fax: +33(0)-15369-7690
Alfred Vidal-Madjar
Institut d'Astrophysique de Paris (IAP/CNRS), Paris, France
Tel: +33-1-44-32-80-73
E-mail: alfred@iap.fr
Lars Lindberg Christensen
Hubble European Space Agency Information Centre, Garching, Germany
Tel: +49-89-3200-6306 (089 within Germany)
Cellular (24 hr): +49-173-3872-621 (0173 within Germany)
E-mail: lars@eso.org
Ray Villard
Space Telescope Science Institute, Baltimore, United States
Tel: +1-410-338-4514
E-mail: villard@stsci.edu